This invention relates to a seat belt apparatus used in a vehicle such as an automobile. More particularly, the invention relates to a pretensioner which, at the moment the vehicle sustains a collision, tightens a seat belt in a direction which restrains the body of the passenger wearing the seat belt.
A seat belt apparatus installed at the seat of an automobile functions to restrain the body of the passenger wearing the seat belt in an emergency situation such as when the vehicle sustains a collision, thereby protecting the passenger from an injury caused by striking the body of the vehicle.
When the passenger is restrained by such a seat belt apparatus, the restraining action diminishes unless the seat belt is so fastened as not to separate from the body of the passenger, i.e., in such a manner that a gap will not form between the belt and the passenger's body. The reason for this is that the amount of movement of the passenger's body due to inertia at the moment of impact increases by an amount equivalent to the separation between the passenger s body and the seat belt, and hence there is the danger that the passenger will strike the vehicle body.
Accordingly, a retractor equipped with a pretensioner has been proposed in view of the foregoing problem. With such a retractor, a passenger's body is restrained by causing a reel to rotate by a pretensioner in a direction that takes up the seat belt on the reel at the moment of a collision, thereby minimizing forward movement of the passenger when a collision occurs. Such a retractor is capable of allowing the seat belt to perform its function to a satisfactory extent.
FIG. 6 is a perspective view of this retractor equipped with a pretensioner according to the prior art. The retractor, shown at numeral 101, includes a real shaft 103 on which a seat belt 102 is so wound as to be freely taken up and paid out, a pulley 104 fixedly secured to one end of the reel shaft 103, a wire rope 105, which comprises a plurality of twisted metal wires, wound several turns on the outer periphery of the pulley 104 with a prescribed clearance therebetween, and tensioning means 106 for tensioning the wire rope 105.
With seat belt retractor 101 having the pretensioner constructed as set forth above, the tensioning means 106 does not operate when the vehicle is traveling in ordinary fashion, namely in a manner in which the only deceleration that can be produced is less than a prescribed value. As a result, the wire rope 105 is not tensioned and the prescribed clearance exists between the wire rope 105 and the pulley 104 at such time. Accordingly, the seat belt 102 can be pulled freely from the retractor 101.
When the vehicle sustains a predetermined degree of deceleration, a deceleration sensor (not shown) senses this deceleration and produces a commensurate output signal, in response to which the tensioning means 106 is actuated to rapidly tension the wire rope 105 in the direction A in FIG. 6. The rapidly tensioned wire rope 105 is taken up on the pulley 104 as the clearance between the two quickly vanishes, and the pulley 104 is rapidly rotated in the direction B owing to a frictional force between the pulley 104 and wire rope 105. Since this rotation of the pulley 104 is accompanied by rotation of the reel shaft 103 also in the direction B, the seat belt 102 is taken up on the reel. Consequently, the seat belt 102 is pulled back to eliminate the gap between the seat belt and the passenger's body, namely the slack in the seat belt 102.
Since the seat belt retractor 101 equipped with the pretensioner having this construction is capable of rapidly tightening the seat belt 102 about the passenger's body at the moment of impact, the retractor is considered to be effective in preventing a decline in the restraining action of the seat belt.
In the pretensioner of the seat belt retractor 101 of this kind, the arrangement is such that the wire rope 105 tensioned by the tensioning means is wound up on the pulley 104, which is an integral part of the reel, when the pretensioner operates. Accordingly, owing to the friction between the wire rope 105 and pulley 104, a force is transmitted from the wire rope 105 to the pulley 104.
However, in this configuration where force is transmitted by means of friction, slip always develops between the wire rope 105 and the pulley 104. As a consequence, the seat belt 102 cannot be tightened reliably and the amount of tightening is not constant. Though it can be arranged to predict the amount of slip as an expedient for dealing with this problem, the degree of tightening will no longer be adequate if the amount of slip exceeds the predicted value thereof, and tightening will be excessive if the amount of slip is less than the predicted value.
Thus, with the conventional pretensioner, tightening of the seat belt 102 cannot be performed reliably. Moreover, the amount of tightening of the seat belt 102 changes depending upon the magnitude of the friction between the wire rope 105 and the pulley 104.